If you want to add highlighting for a language to RSyntaxTextArea that's not built-in, you can either download and use TokenMakerMaker from Subversion, or write your own JFlex-based parser, probably using a similar, already implemented language as a baseline, and follow these instructions to use it at runtime.

But if you feel that there's a language out there that would really benefit from being added to the core, place your vote here! If some language gets a lot of votes (bonus points if there's an actual "IDE"-ish project that would actually use it), then it might get added.

Of course, if you implement a TokenMaker for a new language and would like to contribute it to the core, feel free to get in touch with me (email me, PM, or post here). I'm happy to add support for languages that are fairly well-known/used, but am reluctant to add niche languages with very little usage, to keep the size of the library relatively in check (I'm old-school). Remember that you can always add support for a language to your own project using RSTA at runtime (see above). Any contributions would need to be license-compatible with the project (i.e. modified BSD).

EDIT: LaTeX and DTD used to be options, but they've been added as of 2.0.3, so I've removed them from the poll. Unfortunately this cleared the vote count for all other choices, so feel free to vote again! LaTeX was by far the most popular choice so far, and it now has both syntax highlighting and code folding support. DTD highlighting didn't have as many votes, but I wanted it anyway.

Not sure if this poll is still active, but I would LOVE first class support for Beanshell, as opposed to simply using Java and getting halfway there. That is, I would love first class support for Beanshell AutoComplete and RSTALanguageSupport.